OSB or plywood — which should I use for a floor deck or roof?

What the OSB and plywood grades mean structurally, how each behaves when it gets wet, and the jobs where one is clearly right.

Difficulty: beginner
Time: 15 min
Read: 4 min

Short answer

OSB/3 is the standard, cheaper choice for floor decks, roof decks and sheathing in dry or humid conditions, and it is what most new build uses. Plywood is stiffer, holds screws better at the edges, cuts and finishes far more cleanly, and structural exterior-grade plywood copes better with repeated wetting.

They occupy the same place in a merchant and the same place in a specification, and the choice usually comes down to cost against edge quality and wet performance.

Step by step

Check the grade rather than the material name.OSB comes as OSB/2 for dry conditions and OSB/3 for humid; OSB/4 is heavy duty. Plywood is graded by bond class, with exterior grades using a weather and boil proof adhesive.
Use OSB/3 for structural decks in normal conditions.Floor decks, flat roof decks and wall sheathing. It is what most volume housebuilding uses, it is cheaper, and it performs well when kept dry in service.
Use plywood where edges and fixings matter.Plywood holds screws near an edge far better than OSB, whose strands can break away. For anything screwed close to an edge, plywood is the sounder choice.
Use plywood where the surface will be seen or worked.OSB cannot be planed, routed or finished neatly, and its edges cannot be made crisp. Plywood machines cleanly and takes an edge banding.
Think hard about water.Both swell if soaked. OSB swells irreversibly at the edges and the strands lift; the swelling never goes back down. Exterior-grade plywood recovers better and delaminates only if the bond fails.
Consider stiffness.Plywood is generally stiffer for a given thickness, so for spanning between joists at wider centres it may allow a thinner board.
Use tongue-and-groove boards on floors.T&G OSB or plywood locks adjacent boards together, which removes squeaks and steps at the joints and means you do not have to land every edge on a joist.
Match the thickness to the joist spacing.Eighteen millimetre boards on joists at 400mm centres is the typical domestic floor deck. Wider centres need thicker boards, and the manufacturer publishes span tables.
Seal cut edges on either material outdoors.Cut edges expose the core to water. A coat of exterior primer or sealer on every cut edge makes a large difference to how long the board survives.
Consider the weight and handling.Both are heavy in 2400 by 1200 sheets. OSB is slightly lighter for the same thickness, and neither is a one-person lift up a ladder.

Tips

OSB edge swelling is permanent. Keep it dry on site and out of standing water, especially before the roof is on.
For a shed or workshop floor, tongue-and-groove OSB/3 is the sensible economical choice.
For anything screwed near an edge, or anything you will see, use plywood.

Common mistakes

Using OSB/2 where it can get damp — OSB/2 is specified for dry conditions only. Used in a humid or occasionally wet situation the bond degrades and the board loses strength as well as swelling.
Assuming any plywood is exterior grade — Plywood bond classes vary enormously. Interior-grade ply delaminates within a season outdoors, and the face grade tells you nothing about the glue.

If it doesn't work

OSB floor deck swollen along the joints

Cause: Water got onto the deck before it was weatherproof — Fix: Swelling is not reversible. Sand or plane the raised edges flat if it is minor; replace affected boards if it is significant.

Screws pulling through near the board edge

Cause: OSB strands breaking away at the edge — Fix: Move fixings further from the edge, use screws with a larger head, or use plywood where edge fixing is unavoidable.

Plywood delaminating outdoors

Cause: Interior-grade bond — Fix: Replace with a board carrying an exterior or marine bond class, and seal all cut edges.

Bouncy floor despite new decking

Cause: Board too thin for the joist spacing, or joists themselves deflecting — Fix: Check the board thickness against the span table for the actual joist centres, and assess the joists separately.

Questions people ask

Is OSB structurally as good as plywood?

For panel strength and racking resistance, OSB/3 is a recognised structural board and is used throughout modern construction. Plywood is stiffer and better in point loading and edge fixing.

Can I use either under tiles?

Plywood of adequate thickness is the conventional base for tiling, well fixed to reduce deflection, and many tilers prefer a cement board over it. OSB is generally not recommended as a tile substrate.

Which is better for a shed floor?

OSB/3 on a dry, ventilated base is cheap and perfectly adequate. Keep it clear of the ground, ventilate underneath, and seal the edges.